The present invention relates to an optical disk player, and more particularly to a light signal detecting circuit for a magnetooptical disk player.
In general, a magnetooptical disk player reproduces recorded information by irradiating laser beam focused in a minute spot with the diameter of about 1 .mu.m onto the magnetooptical disk, then detecting the information recorded in the magnetization direction of magnetooptical material by converting the information to the rotation of polarized surface according to Kerr or Faraday effect that explains the interaction between light and magnetization.
Referring to FIG. 1, such magnetooptical disk player irradiates laser beam from a semiconductor laser diode 2 onto a magnetooptical disk 6, and detects light reflected from the magnetooptical disk by converting the rotation variation of the polarized surface to light intensity variation by an analyzer such as polarizing beam splitter (PBS). In other words, a differential light detector detects the difference of light intensity between polarized component passing through the PBS and polarized component reflected from the PBS to output reproduced data signal. Accordingly, since the conventional magnetooptical disk player detects a signal by S/N ratio, the improvement of the S/N ratio means the improvement of reproduction efficiency. As shown in FIG. 2, the magnetooptical disk player has light intensity noise of semiconductor, short noise of light detector caused by PN junction, and thermal noise of signal amplifier, which are problems to solve.
The light intensity noise of semiconductor laser has a close relation with an oscillate mode of laser. Thus, recently, a single mode semiconductor laser diode is driven by a high frequency signal of hundreds of MHz so as to operate the diode as a multimode diode which reduces the semiconductor laser noise. However, since the short noise of light detector is relatively greater than the thermal noise of signal amplifier, a finally output S/N value is greatly affected by the short noise.